1. Field of the Invention
The present invention relates to silicide/silicon carbide composites and articles of manufacture based thereon.
2. Discussion of the Prior Art
Silicides of transition metals such as molybdenum disilicide (MoSi.sub.2) and composites comprising matrices of the transition metal silicide reinforced with silicon carbide (SiC) are considered valuable materials for high-temperature structural applications due to their high melting point, excellent oxidation and corrosion resistance, low density and good electrical and thermal conductivity properties. Similar to many other high-temperature intermetallics, the use of MoSi.sub.2 is limited as a structural material due to its low ambient temperature fracture toughness and poor elevated temperature strength.
A number of approaches for the processing of this intermetallic are unsuitable due to its high melting point and owing to the fact that it exists as a line compound. Furthermore, the relatively high dissociation pressures of silicides such as MoSi.sub.2 at elevated temperatures result in uncontrolled second phase formation due to silicon volatilization [T. G. Chart, Metal Science, Vol. 8, p. 344 (1974); and Searcy et al, J. Phys. Chem., Vol. 64, p. 1539 (1960)]. In view of these characteristics, powder processing has been the preferred fabrication route due to the lower processing temperatures that it affords; unfortunately, it also results in the incorporation of silica (originally formed as a surface layer on the powder particles [Berkowitz-Mattuck et al, Trans. Metall. Soc. AIME, Vol. 233, p. 1093 (1965)]) into the consolidated samples. The presence of grain boundary silica either as a continuous film or as discrete particles is detrimental since the particles may serve as crack nucleation sites at lower temperatures, while enhancing deformation via grain boundary sliding at temperatures about 1,200.degree. C. where the silica softens appreciably. In fact, recent studies have shown that low silica polycrystalline MoSi.sub.2 demonstrates negligible "plasticity" below 1,400.degree. C. [Aikin, Jr., Scripta Metall., Vol. 26, pp. 1025-1030 (1992)]. Silica formation also alters the matrix stoichiometry and results in the formation of Mo.sub.5 Si.sub.3. Such stoichiometric deviations degrade the intermediate temperature oxidation resistance [Meschter, Metall. Trans., Vol. 23A, pp. 1763-1772 (1992)] of the silicide. Finally, silica has also been reported to cause the degradation of the diffusion barrier coatings at the fiber-matrix interface in ductile fiber-reinforced MoSi.sub.2 [Xiao et al, Mater. Sci. Eng., Vol. A155, p. 135 (1992)].
In attempting to control the oxygen content of MoSi.sub.2 by varying the starting powder size and by intentional carbon additions (deoxidant), Maxwell [NACA RM E52B06 (1952)] found that a fine-grained material with carbon additions had better creep properties and lower high-temperature plasticity than a similar grain-size material without carbon. More recently, Maloy et al [J. Am. Ceram. Soc., Vol. 74, p. 2704 (1991)] also reported improved elevated temperature fracture toughness with varying levels of carbon additions. However, substantial (.about.40%) weight losses were reported on consolidating these samples, resulting in formation of Mo-rich second phases. Hardwick et al [Scripta Metall., Vol. 27, p. 391 (1992)] attempted to process oxygen-free MoSi.sub.2 by conducting all the powder handling and consolidation steps under vacuum or inert gas atmospheres. However, these approaches [Hardwick et al, supra; and Schwarz et al, Mater. Sci. Eng., Vol. A155, p. 75 (1992)] are impractical from the standpoint of processing bulk structural parts due to the difficulties involved in the scale-up of the evacuation systems, as well as the excessive costs that would be associated with such processes.
Other methods for forming MoSi.sub.2 /SiC composites are described in U.S. Pat. Nos. 4,927,792 and 5,000,896.
Methods for the production of alloy silicide/SiC composites are described in U.S. Pat. Nos. 4,970,179 and 5,069,841.
It is, therefore, clear that further enhancements in the properties of MoSi.sub.2 and MoSi.sub.2 alloy silicide based composites are possible only with the elimination of silica (and oxygen) in the matrix, along with close control of the overall stoichiometry, through the use of simple and economical processing schemes that do not necessitate elaborate care during powder handling.
It is an object of the present invention to provide novel, substantially silica-free, silicide/SiC composites such as MoSi.sub.2 /SiC, methods for their production and articles of manufacture derived therefrom which are not subject to the above-noted disadvantages.
It is another object of the invention to form a composite material of uniformly dispersed particles of silicon carbide in a silicide or an alloy silicide matrix, particularly molybdenum disilicide. The matrix contains alloy silicides such as (Mo,W)Si.sub.2. The volume fraction of the silicon carbide can be varied from 10% to 90%.
It is also an object of the invention to provide enhanced high-temperature properties to silicide matrices, especially above the softening temperature of the silica, through the elimination of the siliceous intergranular phase and its conversion to stable silicon carbide through in situ carbothermal reduction reactions.